This invention relates to apparatus for pulverizing, i.e., grinding, material, and more specifically to a coiled spring system that is particularly suited for embodiment in a bowl mill wherein the coiled spring system is operative for purposes of establishing the magnitude of the forces which the grinding rolls of the bowl mill exert on the coal for purposes of effectuating the pulverization of the coal.
An essential component of any steam generation system of the type, which utilizes pulverized coal as a fuel, is the apparatus in which the coal is pulverized so as to render it suitable for such usage. Although the prior art is known to have employed various types of apparatus for purposes of accomplishing coal pulverization, one form of apparatus in particular, which has frequently been used for this purpose, is that commonly referred to as a bowl mill by those in the industry. The bowl mill obtains its name principally from the fact that the pulverization, i.e., grinding, of the coal that takes place therewithin occurs on a grinding surface which in configuration somewhat resembles a bowl.
By way of illustration, reference may be had to U.S. Pat. No. 3,465,971 which issued Sept. 9, 1969 to J. F. Dalenberg et al. and which is assigned to the same assignee as the present invention for a showing of a prior art form of bowl mill. This patent contains a teaching of both the nature of the construction and the mode of operation of a bowl mill that is suitable for use for purposes of effectuating the pulverization of the coal that is used to fuel a coal-fired steam generator. As taught by this patent, the essential components of such a bowl mill are a body portion, i.e., housing, within which a grinding table is mounted for rotation, a plurality of grinding rolls that are supported in equally spaced relation one to another in a manner so as to coact with the grinding table such that the coal disposed on the surface of the grinding table is capable of being ground, i.e., pulverized, by the rolls, coal supply means for feeding to the surface of the grinding table the coal that is to be pulverized in the bowl mill, and air supply means for providing to the interior of the body portion the air that is required for the operation of the bowl mill.
In order to satisfy the demands of a coal-fired steam generation system of conventional construction for pulverized coal a multiplicity of bowl mills of the type shown in the aforereferenced patent are commonly required to be employed. Further in this regard, it is noted that the individual capacity of each of these bowl mills may range up to a capacity of one hundred tons of pulverized coal per hour. In addition to possessing a capability of operating at their maximum capacity, these bowl mills must also have the ability to operate at less than full capacity, i.e., at some percentage thereof, e.g., 25%, 50%, 75%, etc. Accordingly, this fosters a further requirement that the bowl mill be capable of exerting the requisite degree of grinding force regardless of the rate of output at which the bowl mill is operating. Here note is taken of the fact that variations in the output provided from the bowl mill are normally accomplished by varying the amount of coal that is fed to the grinding table, while the speed of rotation of the grinding table is made to remain substantially constant.
The depth of coal that is disposed on the grinding table is a function of the output rate at which the bowl mill is performing. In addition, the depth of coal that is present on the grinding table has an effect on the amount of grinding force being exerted on the coal by the grinding rolls. Obviously, therefore, it is important that if the grinding rolls are to apply the requisite degree of force needed to effect the pulverization of the coal, consideration must be given to the existence of this relationship between the grinding force exerted by the grinding rolls and the depth of coal on the grinding table.
The journal loading, which dictates the amount of grinding force that the grinding rolls exert on the coal, has heretodate been provided either through the use of hydraulic systems or through the use of mechanical springs. One such arrangement of mechanical springs can be found depicted, for example, in the patent which was referred to above previously. In accord with the showing contained in this U.S. patent, each grinding roll is urged towards the surface of the grinding table by means of an adjustable spring. To this end, there is selected for use for this purpose, a mechanical coil spring that possesses the design characteristics desired; namely, a spring that is capable of urging the grinding roll toward the grinding table surface in such a manner that the grinding roll exerts a predetermined grinding force on the coal disposed on the table, when the coal is of a predetermined depth on the table.
As an alternative to the use of mechanical coil springs for purposes of providing the journal loading, the prior art on occasion has turned to the employment of hydraulic systems. U.S. Pat. No. 4,002,299, which issued on Jan. 11, 1977 to C. J. Skalka and which is assigned to the same assignee as the present application, is directed to one arrangement of such a hydraulic system. In accord with the teachings of this patent, a system is provided wherein the grinding rolls have a hydraulic loading applied thereto. More specifically, the hydraulic loading on the grinding rolls is established by means of hydraulic fluid that is fed under pressure to the grinding rolls. Moreover, through the use of a servo system, changes in the hydraulic pressure are automatically effected as the mill output increases or decreases.
Notwithstanding the fact that hydraulic systems have been utilized heretofore for purposes of accomplishing the journal loading of the grinding rolls in a bowl mill, when so employed for this purpose such hydraulic systems and in particular such hydraulic systems that make use of a servo system have been known to be disadvantageously characterized. More specifically, reference is had here to the fact that a characteristic of the servo systems employed in connection with the establishment of the hydraulic loading on the grinding rolls in bowl mills is the susceptibility of such servo systems to the phenomenon known as hunting. Inasmuch as those skilled in the art of servo systems are well acquainted with this phenomenon, it is not deemed necessary to discuss it at length herein. Rather, it is deemed sufficient to merely note that hunting is that phenomenon wherein the servo system in its attempt to effect the establishment of the proper hydraulic pressure continually signals the need for minor corrective adjustments to be made in the hydraulic pressure. To this end, each time the servo system senses a deviation from the desired pressure level, it signals the need for corrective action to be taken. This corrective action instituted by the servo system in turn elicits from the latter the need for a further change. This process, which may go on ad infinitum, is what is referred to herein as the phenomenon of hunting.
In addition to being disadvantageously characterized by virtue of their susceptibility to hunting, as discussed above, the systems that have been suggested to date by the prior art for purposes of hydraulically loading the grinding rolls in a bowl mill have in general also suffered from a further disadvantage. Reference is had here to the fact that most, if not all, of the hydraulic systems that have been suggested for use by the prior art in connection with establishing the loading on the grinding rolls in a bowl mill are unsuitable for use for purposes of effecting changes in the amount of grindng force that the rolls exert. That is, the mode of operation of these prior art forms of hydraulic systems is such that they are intended to ensure that a fixed value of hydraulic pressure is continually applied to the grinding rolls in the form of the journal loading thereon.
The difficulty arises here from the fact that although a particular value of hydraulic pressure may be selected so as to cause the grinding rolls to exert the optimum amount of grinding force for a particular set of operating parameters, as the latter parameters vary in the course of the operation of the bowl mill, the value of the pressure of the hydraulic fluid being fed to the grinding rolls may not necessarily be the same as that which should be present to ensure that the grinding rolls are still exerting the optimum amount of grinding force under this changed set of operating parameters. Moreover, once the value of the pressure of the hydraulic fluid that is to be supplied to the grinding rolls is established, in accord with the mode of operation of most, if not all, of these prior art forms of hydraulic systems, this value for the hydraulic pressure cannot be changed. That is, changes cannot be effected in the established value for the pressure of the hydraulic fluid in accordance with the need to vary the amount of grinding force that the grinding rolls are required to exert in order to pulverize to the desired extent the coal that is disposed on the grinding table surface.
A need has thus existed in the prior art for a new and improved means, preferably of the mechanical spring type, for providing the journal loading on the grinding rolls of a bowl mill. Moreover, a need has been demonstrated for such a journal spring assembly which could be provided in the form of a completely subassembled unit that could be assembled, preloaded and stored as a spare part. In addition, a need has been shown for such a journal spring assembly which would not suffer from the same difficulties that have served to disadvantageously characterize the operations of bowl mills that have been equipped with prior art forms of mechanical coil spring journal assemblies such as the fact that such prior art forms of mechanical coil spring journal assemblies have demonstrated a susceptibility to being adversly affected by the operating conditions which exist within the bowl mill. Finally, a need has been evidenced in the prior art for such a journal spring assembly which in addition to embodying the beneficial attributes enumerated above would also be advantageously characterized by the fact that the subject journal spring assembly would be simpler in construction and easier to employ while yet being capable of providing reliable operation relatively inexpensively.
It is, therefore, an object of the present invention to provide a new and improved assembly operable for establishing the journal loading on the grinding rolls of a bowl mill that is suitable for use to pulverize coal.
It is another object of the present invention to provide such an assembly that is operative to establish a mechanical spring journal loading on the grinding rolls of a bowl mill that is suitable for use to pulverize coal.
It is still another object of the present invention to provide such a mechanical spring assembly for establishing the journal loading on the grinding rolls of a bowl mill which is characterized by the fact that the mechanical spring assembly can be provided in the form of a completely subassembled unit which can be assembled, preloaded and stored as a spare part.
A further object of the present invention is to provide such a mechanical spring assembly for establishing the journal loading on the grinding rolls of a bowl mill that is characterized by the fact that the spring portion thereof is suitably positioned such that minimal influence is exerted thereupon by the operating conditions which exist within the bowl mill.
A still further object of the present invention is to provide such a mechanical spring assembly for establishing the journal loading on the grinding rolls of a bowl mill which is characterized by the fact that the mechanical spring assembly is simpler in construction than prior art forms of mechanical spring journal assemblies.
Yet another object of the present invention is to provide such a mechanical spring assembly for establishing the journal loading on the grinding rolls of a bowl mill which is characterized by the fact that the mechanical spring assembly is easier to employ than prior art forms of mechanical spring journal assemblies.
Yet still another object of the present invention is to provide such a mechanical spring assembly for establishing the journal loading on the grinding rolls of a bowl mill which is characterized by the fact that the mechanical spring assembly is capable of furnishing reliable operation while yet being relatively inexpensive to provide.